This invention pertains to a cathode-ray tube with an improved internal magnetic shield.
A color cathode-ray tube typically has an internal magnetic shield to reduce the influence of magnetic fields on electron beam trajectories as a cathodoluminescent screen of the tube is scanned. The shield is usually made of 0.10 to 0.18 mm thick cold-rolled steel and is fastened to a shadow-mask frame so that the shield and frame are magnetically coupled. The frame is supported by mounting studs that extend inwardly from a glass rectangular faceplate panel of the tube. During tube fabrication, the magnetic shield is fastened to the frame prior to the step of frit sealing an edge of the faceplate panel to a glass funnel of the cathode-ray tube. The magnetic shield is designed to fit into the funnel and be as close to the funnel wall as possible so that it will not provide a surface from which overscan electrons will be scattered back onto the screen. The magnetic shield should not touch the funnel of the cathode-ray tube to avoid any friction between the shield and a conductive anode coating on the inner surface of the glass funnel. Such friction may occur during assembly of the funnel to the faceplate panel or during uneven expansion at a subsequent heating step, and would create loose particles within the tube which are undesirable. The conductive anode coating is generally applied so that it extends to within a predetermined distance from the glass frit applied between the funnel and the panel, in order to avoid possible contamination thereof during the frit sealing step.
The cathodoluminescent screen of the cathode-ray tube generally comprises a thin coating of aluminum on the back surface of a phosphor screen. This aluminized coating is applied to the inner surface of the glass faceplate panel so that it extends to within a predetermined distance from the glass frit, also to avoid possible contamination of the glass frit. The aluminum coating is connected to the conductive anode coating inside the funnel by means of spring attachments connected to the frame after the magnetic shield is fastened thereto. As a result, the full anode voltage is applied to the phosphor screen. However, the inner glass surfaces of the funnel and the panel adjacent the edge thereof are left uncoated.
During warmup of the color cathode-ray tube in a television receiver, bright areas have been observed in corners thereof which remain visible throughout the warmup period, particularly in the upper two corners. It was observed that the placement of mounting hardware or grounding straps enhances the intensity of the bright areas, and that the brightness effect is more pronounced with an electron beam overscan greater than 5 percent, particularly at 10 percent overscan which is common in most tubes. The bright areas are a time oriented effect which disappear after a few seconds or minutes after the warmup period. The problem exists on all tube types utilizing an internal magnetic shield. However, the effect of this problem was heightened after a modification was made to the design of an internal magnetic shield to provide more clearance between the shield and the funnel. The diagonal contours of this internal magnetic shield were flatened to provide more clearance from the shield to the conductive anode coating on the inside surface of the funnel. The present invention provides for an apertured internal magnetic shield which eliminates the bright areas noticed in the corners of the cathode-ray tube during warmup.